The present invention relates to a method of manufacturing a spline telescopic shaft.
JP-A-2011-38561 discloses that an inner shaft manufacturing intermediate member having male splines on which a first resin coating is formed, and an outer shaft manufacturing intermediate member in which female splines are formed are forcibly reciprocally slid in the axial direction under given conditions, to transfer apart of the resin material of the first resin coating to the female splines of the outer shaft manufacturing intermediate member, thereby forming a second resin coating.
Conditions that the sliding load in the axial direction is not larger than a predetermined value, and that the play in the rotation direction is not larger than a predetermined amount are sometimes set as performances required in a spline telescopic shaft which is used as, for example, a steering shaft or intermediate shaft in a vehicle.
Usually, there is a tendency that, when the sliding load is excessively reduced, the play in the rotation direction is large, and, when the play in the rotation direction is excessively reduced, conversely, the sliding load is large. Namely, it is difficult that the two performances are compatible.
It is assumed that the sliding step disclosed in JP-A-2011-38561 is performed under given conditions (for examples, conditions that the sliding step is ended at a timing when the sliding load reaches a predetermined level). In this case, when spline telescopic shafts are mass-produced, there is a possibility that one of the two performances (the sliding load and the play) becomes faulty due to a failure of the tooth profiles of the splines, thereby causing the failure rate to be high.